Ultrasonic waves propagate through various mediums including gas, liquid and solid mediums and so are used in various fields including measurement, physical property measurement, engineering, medicine and biology.
Ease of propagation of an ultrasonic wave at an interface between different mediums is represented as an acoustic impedance ratio. In general, an ultrasonic wave is mostly reflected by an interface between mediums having significantly different levels of acoustic impedance, such as an interface between a gas and a solid, and cannot propagate to a different medium at high efficiency.
For detecting an ultrasonic wave, an ultrasonic vibrator is widely used, which includes a piezoelectric element formed of ceramics or the like. Therefore, when an ultrasonic wave which has propagated through a gas is to be detected by an ultrasonic vibrator, the ultrasonic wave which has propagated is mostly reflected by a surface of the ultrasonic vibrator and is only partially detected by the ultrasonic vibrator. This makes it generally difficult to detect an ultrasonic wave at high sensitivity. When an ultrasonic wave is transmitted from an ultrasonic vibrator to a gas also, the propagation efficiency is reduced by the reflection. Accordingly, when using an ultrasonic wave specifically for measuring a distance or flow rate or for detecting physical properties, how to detect an ultrasonic wave at high sensitivity is one of important issues.
In order to solve this problem, Patent Document 1, for example, discloses an ultrasonic transmitter/receiver main body capable of transmitting and receiving an ultrasonic wave at high efficiency in a wide band, using refraction of an ultrasonic wave in a gas. Hereinafter, this ultrasonic transmitter/receiver main body will be described.
As shown in FIG. 8, this conventional ultrasonic transmitter/receiver main body 101 includes an ultrasonic vibrator 2 and a propagation medium section 3 provided on a wave receiving face of the ultrasonic vibrator 2. A space around the ultrasonic transmitter/receiver main body 101 is filled with, for example, a fluid 4, for example, air. An interface between the ultrasonic vibrator 2 and the propagation medium section 3 will be referred to as a first surface area 31, and an interface between the propagation medium section 3 and the fluid 4 will be referred to as a second surface area 32. An angle made by the first surface area 31 and the second surface area 32 is represented with θ1, and an angle made by the normal to the second surface area 32 and a traveling direction of an ultrasonic wave is represented with θ2. X, Y and Z directions are set as shown in FIG. 8.
An ultrasonic wave is transmitted as follows. An electric signal is given to the ultrasonic vibrator 2 from a driving circuit (not shown), and the ultrasonic vibrator 2 is vibrated to generate an ultrasonic wave. The ultrasonic wave generated in the ultrasonic vibrator 2 propagates from the first surface area 31 toward the second surface area 32 through the transmission medium section 3 in a positive Y axial direction. Upon arriving at the second surface area 32, the ultrasonic wave changes the propagation direction thereof in conformity to the law of refraction, and propagates in the direction of an ultrasonic transmission path 5 in the fluid 4.
An ultrasonic wave is received as follows, i.e., oppositely to the manner of transmission. The ultrasonic wave, which has propagated through the fluid 4 filling the space around the ultrasonic transmitter/receiver main body 101, reaches the second surface area 32 and is refracted and transmitted through the propagation medium section 3. Then, the ultrasonic wave propagates through the propagation medium section 3 in a negative Y axis direction and reaches the ultrasonic vibrator 2. Upon reaching the ultrasonic vibrator 2, the ultrasonic wave deforms the piezoelectric element of the ultrasonic vibrator 2 to generate a potential difference between the electrodes and is detected by a receiving circuit (not shown).
As described above, the ultrasonic wave is refracted at the interface between the propagation medium section 3 and the fluid 4. Such a type of ultrasonic transmitter/receiver main body is specifically called an “oblique propagation type ultrasonic transmitter/receiver main body”. In the ultrasonic transmitter/receiver main body 101, even where the fluid 4 is a medium having a very low level of acoustic impedance (sonic velocity in the medium×density of the medium) such as air or the like, the ultrasonic wave can be incident on the propagation medium section 3 from the fluid 4 at high efficiency, or can go out from the propagation medium section 3 to the fluid 4 at high efficiency.
Where the sonic velocities of the ultrasonic wave in the propagation medium section 3 and the fluid 4 are C1 and C2, and the densities of the propagation medium section 3 and the fluid 4 are ρ1 and ρ2, reflectance R of the ultrasonic wave at the interface between the second surface area 32 and the fluid 4 is represented by the following expression (1).
                              [                      Expression            ⁢                                                  ⁢            1                    ]                ⁢                                  ⁢                                  ⁢                  R          =                                                                      ρ                  2                                                  ρ                  1                                            -                                                tan                  ⁢                                                                          ⁢                                      θ                    1                                                                    tan                  ⁢                                                                          ⁢                                      θ                    2                                                                                                                        ρ                  2                                                  ρ                  1                                            +                                                tan                  ⁢                                                                          ⁢                                      θ                    1                                                                    tan                  ⁢                                                                          ⁢                                      θ                    2                                                                                                          (        1        )            In the case where C1, C2, ρ1 and ρ2 fulfill the following expression (2), the values of θ1 and θ2 with which the numerator of expression (1) is zero necessarily exist. Namely, the reflectance R is zero.
                    [                  Expression          ⁢                                          ⁢          2                ]                                                                                      ⁢                                            ρ              2                                      ρ              1                                <                                    C              1                                      C              2                                <          1                                    (        2        )            θ1 and θ2 fulfill expression (3) (Snell's law).
                    [                  Expression          ⁢                                          ⁢          3                ]                                                                                      ⁢                                            sin              ⁢                                                          ⁢                              θ                1                                                    C              1                                =                                    sin              ⁢                                                          ⁢                              θ                2                                                    C              2                                                          (        3        )            As a condition for θ1 under which the reflectance R is zero, expression (4) is obtained using expression (3).
                    [                  Expression          ⁢                                          ⁢          4                ]                                                                                      ⁢                                            tan              2                        ⁢                          θ              1                                =                                                                      (                                                            ρ                      2                                                              ρ                      1                                                        )                                2                            -                                                (                                                            C                      1                                                              C                      2                                                        )                                2                                                                                      (                                                            C                      1                                                              C                      2                                                        )                                2                            -              1                                                          (        4        )            
Namely, as shown in Patent Document 1, when expression (2) is fulfilled in the oblique propagation type ultrasonic transmitter/receiver main body, there exists a direction (angle θ1) in which the transmission efficiency of the ultrasonic wave at the second surface area 32 can be approximately 1. The angle θ1 made by the first surface area 31 and the second surface area 32 at this point is represented by expression (4). Expressions (1) and (4) do not heavily rely on the frequency of the propagating ultrasonic wave. Therefore, the oblique propagation type ultrasonic transmitter/receiver main body 101 capable of transmitting and receiving an ultrasonic wave at high efficiency and in a wide band is realized.
Patent Document 1: United States Laid-Open Patent Publication No. 2005/0139013
Patent Document 2: Japanese Laid-Open Utility Model Publication No. 58-195884
Patent Document 3: Japanese Laid-Open Patent Publication No. 5-292598